A well known radio communications system is a trunked radio system, such as that used in land mobile service. Such a system typically includes several transmitters and receivers, or so-called transceivers, implemented as repeaters that are located throughout a geographical coverage area of the system. Thus, communication among radios--e.g., mobiles, portables--throughout the coverage area is facilitated through use of the repeaters located throughout the coverage area.
In today's radio communications systems, signals that are exchanged between two communication devices--e.g., a repeater and a radio--transmitted over an intervening air space, typically suffer from a degradation in quality before being received. Over some design range, the system might function within acceptable parameters, such that the degradation falls within a pre-established design limit. However, when this design range is exceeded, the quality of the transmitted signal falls below the acceptable quality level, and satisfactory communication can not be successfully maintained.
The aforementioned unfavorable condition--i.e., unacceptable signal quality--may be the result of the spatial distance between the transmitter and receiver--i.e., the actual distance being greater than some preestablished design range. Alternatively, the poor quality might come about due to obstructions--e.g., buildings, hills--in the signal path between the transmitter and receiver. Further, the degradation may be the result of a change in the direction of propagation of the signal--e.g., reflection and/or diffraction upon and around buildings.
Regardless of the source of this unfavorable condition--i.e., poor signal quality--it is possible to improve the quality of the signal by various known means. Generally, diversity techniques--e.g., frequency-, time-, and space-diversity employed in the communications resource--are used to improve the communications quality of the signal. However, these techniques have both theoretical and practical limitations, as later described.
One method used to improve the quality of the signal beyond the range given is to increase the signal power--i.e., signal energy--of the transmitted signal. Likewise, a decrease in the noise power--i.e., noise energy--detected by the receiver will serve to improve the resulting quality of the signal. However, transmission power is limited by the technology available to manufacture power amplifiers, and by the current drain that is permissible from the power supplies. Also, high power amplifiers require a relatively large size because of the required size of the heat sink used to dissipate the heat.
In addition, the federal communications commission (FCC) places strict limits on the power radiated, since high power levels will cause problems in frequency re-use schemes--i.e., the same frequency resource is utilized in nearby geographical regions. At base sites, the industry is close to the limits of RF power imposed by the FCC, and is at the limits of the technology. In the remote transceivers, the current drain and size are limiting greater power. A small improvement in noise figure reduction of the receivers is possible, but the theoretical limit is rapidly being reached.
Diversity techniques utilizing the parameters of frequency, time, space, angle, and polarization in the communications resource are also capable of improving the communications quality of the signal to overcome the limitations described above. However, these improvements do not come without cost. For example, space-, angle-, and polarization-diversity require additional hardware at the receiver. At a minimum, a second antenna and coaxial line--representing an undesirable cost increase--are required. In addition, a second receiver and/or switch may be required, thereby increasing costs and adding other technical limitations. As for time- and frequency-diversity schemes, the associated cost is usually measured in terms of reduced spectral efficiency. That is, by employing a time- and/or frequency-diversity scheme at all times, even when the signal quality is acceptable, the resulting spectral inefficiency may be prohibitive.
Accordingly, there exists a need for a radio communication system that provides improved signal quality, without suffering from the aforementioned shortcomings of the prior art. In particular, a system that provided improved signal quality, without additional hardware costs or unacceptable reductions in bandwidth efficiency, would be an improvement over the prior art.